The present invention relates to an electric discharge lamp apparatus having a structure that a front end of an arc tube is supported by a lead support extending from an insulating plug and a rear end of the arc tube is directly supported by the insulating plug. More particularly, the present invention relates to an electric discharge lamp apparatus incorporating an arc tube which has a structure that an ultraviolet-ray-shielding globe for surrounding and enclosing an enclosed glass bulb is integrally welded to the body of an arc tube having an enclosed glass bulb in which electrodes are disposed opposite to each other, and a rear end of the ultraviolet-ray-shielding globe of the arc tube is supported by a metal support member secured to the insulating plug.
As shown in FIG. 15, a conventional electric discharge lamp apparatus has a structure that a pair of long and short lead supports 2a and 2b projecting over an insulating plug (an insulating base) 1 made of synthetic resin supports the front and rear ends of an arc tube body 5. Electrodes 6 opposite to each other are disposed in an enclosed glass bulb 5a of an arc tube body 5. Pinch seal portions 5b for sealing molybdenum foil members 7 connected to the electrodes 6 are provided for the enclosed glass bulb 5a. Lead wires 8a and 8b connected to the molybdenum foil members 7 are outwards extended from the pinch seal portions 5b. The lead wires 8a and 8b are, directly or through a metal support member 9, secured to lead supports 2a and 2b by welding, the lead supports 2a and 2b being passages for electric power.
The enclosed glass bulb 5a includes metal halide, for example, a sodium-thallium-indium material or a scandium-sodium material. An ultraviolet-ray shielding globe 3 is secured and held by the insulating plug 1 such that the globe 3 surrounds the enclosed glass 5a of the arc tube. The globe 3 cuts ultraviolet rays in wavelength regions of light emitted from the arc tube body 5 which are harmful to the human body or elements constituting the lamp.
However, the metal lead supports 2a and 2b disposed in the ultraviolet-ray shielding globe 3 are exposed to ultraviolet rays emitted in the electric discharge portion of the arc tube. Thus, free electrons having negative electric charges are excited and discharged from the inside portion of the lead support. When the free electrons reach the portion in the vicinity of the outer surface of the enclosed glass bulb 5a, metal atoms (Na+) having positive electric charges in the enclosed glass bulb 5a are pulled by the free electrons having the positive electric charges, and then allowed to pass through the wall of the glass bulb. Thus, the metal atoms are discharged to the outside of the glass bulb. As a result, the steam pressure of light emitting substances in the enclosed glass bulb 5a is lowered. Thus, there arises a problem in that the lifetime of the arc tube is shortened.
Accordingly, an electric discharge lamp apparatus has been suggested which has a structure as shown in FIG. 16 such that the arc tube incorporates the arc tube body 5 to which the ultraviolet-ray shielding globe 3 is integrally welded so as to surround and enclose the enclosed glass bulb 5a. Moreover, the front surface of the insulating plug 1 directly supports the rear end of the arc tube and the outer surface of the globe 3.
A dish-shape disc 4 made of ceramics is secured to the front end of the insulating plug 1 with screws. The rear end of the arc tube penetrates an arc-tube insertion hole 4a formed in the disc 4 so as to be engaged to an engaging hole la formed in the insulating plug 1. Moreover, the globe 3 is held by an inorganic bond layer 4b injected into the disc 4. A screw 4c secures the disc 4 to the insulating plug 1. A focusing ring 1b integrally secures to the outer surface of the insulating plug 1.
The ultraviolet-ray shielding globe 3 cuts ultraviolet rays of light emitted by the enclosed glass bulb 5a in the wavelength regions harmful to the human bodies and the elements constituting the lamp. Therefore, an adverse influence on the human body and the elements constituting the lamp can be eliminated. Moreover, the problem can be prevented which has been experienced with the conventional structure in that free electrons are discharged from the lead support exposed to the ultraviolet rays and thus excited, and, therefore, the steam pressure of the light emitting substances in the enclosed glass bulb 5a is lowered. In FIG. 16, the same elements as those shown in FIG. 15 are given the same reference numerals and the same elements are omitted from description.
The electric discharge lamp apparatus shown in FIG. 16 has the structure that the rear end of the arc tube is supported by the disc 4 secured to the front surface of the insulating plug 1. However, inclination of the arc tube with respect to the disc 4 is substantially inhibited. Therefore, an axis (hereinafter called a "discharge axis") L connecting the opposite electrodes 6 of the arc tube to each other cannot accurately be positioned with respect to the disc 4. Since the ceramic disc 4 integrating the arc tube is secured to the insulating plug 1 with the screws 4c, the positions of the disc 4 and the insulating plug 1 cannot be adjusted in the radial direction. Therefore, the conventional technique encounters a difficulty in accurately positioning the discharge axis L of the arc tube with respect to the central axis of the focusing ring 1b.